Datasheet LT3653 (Analog Devices) - 7

LT3653
OPERATION
Please refer to the Block Diagram. The LT3653 is specifi - An active clamp (not shown) on the VC node provides cally tailored for use with a battery charger power path current limit. The LT3653 is internally compensated with controller. The LT3653 regulates the high voltage input to a pole zero combination. a lower voltage to provide power to the system load and to An external capacitor and internal diode, D2, are used to the single-cell Li-Ion battery charger. High effi ciency in the generate a voltage at the BOOST pin that is higher than battery charger is achieved by keeping the voltage across the input supply. This allows the driver to fully saturate the it low. To accomplish this, the battery charger takes control internal bipolar NPN power switch for effi cient operation. of the LT3653 regulator control node, VC, and overrides The switch driver operates from either V the error amp, G1. The output voltage is regulated by the IN or BOOST to ensure startup. battery charger to a voltage slightly above the battery, typically 300mV. An internal regulator provides power to the control circuitry. This regulator includes input undervoltage and overvoltage HVOK is a status pin which indicates to the charger that a protection which disables switching action when V high voltage input is present and that the LT3653 is ready to IN is less than 7V and greater than 33V, typical. When switching is start providing power to the system load. When the HVOK disabled, the LT3653 safely sustains input voltages up to pin is low, the LT3653 is not switching and the system 60V. Note that while switching is disabled the output will output is not supported by the LT3653 regulator. discharge. The LT3653 is a constant frequency, current mode step Output current limiting is provided via the servo action down regulator. A switch cycle is initiated when the of amplifi er G2. The voltage across the sense resistor, 1.5MHz oscillator enables the RS fl ip fl op, turning on R the internal power switch, Q1. The sense amplifi er (A1) SENSE2, is compared to a voltage programmed by ex- ternal resistor R1 on the I monitors the switch current via the voltage dropped LIM pin. A capacitor averages the inductor ripple current. If the averaged inductor cur- across the current sense resistor RSENSE1. The compara- rent exceeds the programmed value then the V tor compares the amplifi ed current signal with the output C voltage is pulled low, reducing the current in the regulator. The (VC) of the error amplifi er (G1). The switch is turned off output current limit circuit allows for lower current rated when this current exceeds a value determined by the power path components and provides better control of VC voltage. The error amplifi er monitors the VOUT volt- system power dissipation. age through an internal resistor divider and, when not driven externally servos the VC voltage to regulate VOUT. If the VOUT voltage drops, the VC voltage will be driven higher increasing the output current and VOUT voltage. 3653f 7